This invention relates to a toner to be used for development of electrostatic latent images in electrophotography or electrostatic printing, particularly to a capsule toner suited for pressure fixing.
Heretofore, a large number of electrophotographic methods have been known as disclosed in U.S. Pat. No. 2,297,691, Japanese Patent Publication No. 23910/1967 and Japanese Patent Publication No. 24748/1968. Generally speaking, these methods utilize photoconductive materials and comprise the steps of forming an electric latent image on a photoconductive member by various means, subsequently developing the latent image with a toner, optionally transferring the toner image onto a transfer material such as paper, and thereafter fixing by heating, pressure or solvent vapor to give a copy.
The method for sticking the toner onto a material to be fixed thereon by pressurization has been disclosed already in, for example, U.S. Pat. No. 3,269,626 and Japanese Laid-Open Patent Application No. 102624/1973. This method has a large number of advantages such as saving of energy, no pollution, no waiting time for copying due to capability of performing copying by only turning on the power source of the copying machine, no fear of scorching of copies, capability of high speed copying and also simple fixing device.
However, according to such a pressure fixing method of the prior art, no satisfactory fixing characteristic can be obtained unless special treatment is applied on the image supporting member. Besides, the fixing pressure is disadvantageously required to be as high as 200 to 300 kg/cm.sup.2. Further, since the toner material for pressure fixing utilizes generally a soft material, it is inherently poor in pot life, the toner particles may agglomerate with each other on standing, until sometimes they are coalesced or blocked, thus causing undesirable phenomena such as filming, carrier staining, and adhesion to fixing roller. With such a background, a large number of microcapsule toners have been proposed in recent years in efforts to produce an ideal toner having overcome the drawbacks as described above. However, even in those capsule toners, there are still many problems to be solved.
For example, the following difficulties may be encountered.
(1) Adhesion between the core particles and the shell material is poor and the microcapsules are inferior in durability.
(2) In the encapsulation step, the core particles are liable to be encapsulated as they are agglomerated or coalesced or encapsulated products are thereafter coalesced with each other, whereby a microcapsule toner with coarse particle sizes is obtained.
(3) When the phase separation method is employed in the encapsulation step, it is difficult to prevent dissolution of the core material into the continuous phase with higher polarity, with the result that independent core particles are produced along with encapsulated particles. On the other hand, even in the case of employing the spraying method, free particles consisting only of core particles are byproduced in a large amount in addition to microencapsulated particles, similarly as in the phase separation method. Further, the particle size distribution is very broad. The by-produced independent particles will consequently cause staining of sleeve and lowering in image density.
(4) When a conventional method is employed in the encapsulation step, due to insufficient wettability between the core particle surfaces and the shell material, it is difficult to completely coat the core surfaces with the shell material, and defective films will frequently be formed. As the result, for example, the microcapsule toner obtained is poor in pot life, the toner particles are susceptible to coalescing or blocking with each other, whereby filming phenomenon tends to occur on the drum surface. Also, other problems such as susceptibility to staining of carrier and further to adhesion to the fixing roller are caused and they can hardly be overcome completely under the present situation.